The invention relates to a centrifugal pump, in particular for blood in cardiac substitution or assist devices, as generically defined by the preamble to claim 1. In particular, the invention relates to an electrically driven rotary pump of the radial/centrifugal type for permanent implantation in patients with terminal cardiac insufficiency who require mechanical support of their blood circulation.
Blood pumps, especially blood pumps or pumps for other vulnerable fluids, must meet special requirements:
1. High hydraulic efficiency, to keep the heat loss given up to the blood or fluid slight and to keep the energy storing means small.
2. Entirely contactless rotation of the rotor inside a hermetically sealed pump housing, thus precluding any wear, abrasion, and local heat development from mechanical friction.
3. Avoidance of standing eddies and flow stasis zones as well as minimal dwell times of the blood or fluid in the pump, to avoid damage to the fluid and the activation of blood coagulation.
4. In blood pumps, minimizing the cell-damaging shear stresses to which the blood is exposed on passing through the pump.
5. Security against mistakes by eliminating complex sensor-based positional regulations of the rotor while simultaneously reducing energy consumption.
6. Eliminating a drive motor with a supported shaft that is subject to wear.
Blood pumps of conventional design, in which the drive of the rotor is done by an electric motor with a supported shaft that penetrates the pump housing and is provided with a shaft seal are therefore unsuited for permanent implantation. Hermetically sealed housings, through whose wall the pump rotor is set into rotation by means of a magnetic coupling, do eliminate leaks but still require an external electric motor. Furthermore, the pump rotor in the housing must be guided by end journal bearings that are bathed with blood; these bearings wear and from local heating denature blood proteins and are capable of activating the coagulation system, which can lead to emboli from abrasion and clots.
A completely contact-free rotation of the pump rotor in the blood can be achieved by means of passive and active magnet bearings, hydrodynamic slide bearings, or a combination of these principles.
Any possible use of this principle must take Earnshaw's theorem into account, which states that it is not possible to keep a body floating in space in a stable position by means of constant magnetic, electrical, or gravitational fields. Any apparent position of equilibrium is in fact unstable, since the body is in that case at a maximum of potential energy. In at least one axis in space, a stabilizing force acting on the system is therefore required. This force must be all the greater, the farther the body is located from the site of the maximum energy. Conversely, only slight restoring forces are necessary, if the system is located a priori in the vicinity of the unstable equilibrium.
Magnetically supported pump rotors with open blades are described in U.S. Pat. No. 6,227,817. Here, a combination of passive magnet bearings for radial stabilization and sensor-based active axial electromagnetic suspension is described. Besides the complex production, this embodiment requires an elongated gap between the rotor and the housing with only inadequate purging and high energy consumption for the axial stabilization, which must counteract the considerable hydraulic axial shear that is generated by an open impeller.
Blood pumps with complete magnetic suspension are described in European Patent Disclosures EP 0 819 330 B1 and EP 0 860 046 B1. Here, the rotor of the pump is embodied as a rotor of a permanent-magnetically excited electrical synchronous machine. The torque is generated by a revolving, radially engaging electromagnetic stator field, as is the position control of the rotor in the radial direction. Separate control windings of the stator are used for this purpose, which convert the signals of spacing sensors into centering forces by way of electronic closed-loop control circuits. Because of the externally located stators for the drive and positional regulation, this pump requires a relatively large amount of installation space. The stabilization of the other three spatial degrees of freedom that cannot be actively triggered is done by passively acting magnetic reluctance forces. Problems also arise in versions with open impellers because of the high hydrodynamic axial shear, which unavoidably occurs. To overcome them, additional active or passive magnet bearings as well as hydrodynamic aids in the form of nozzles, impact plates, inflow tubes, flow resistors, and sealing gaps are proposed, all of which increase the complexity of the system, lessen its efficiency, create flow stasis zones, induce high shear stresses, and are thus entirely unsuitable for the realization of a blood pump, especially for permanent implantation.
Bearingless blood pumps with magnetic suspension and open impellers are also disclosed in U.S. Pat. No. 6,071,093. However, the transmission of the torque is done here by an axially engaging encircling electromagnetic stator field. The axial rotor position and the tilting of the rotor in the housing are stabilized by a sensor-based electromagnetic feedback by means of actuators, while at the same time passive permanent magnet bearings provide the radial centering. The problems of the axial instability of an open impeller are solved—besides by electromagnetic feedback by means of sensors and actuators—by a fluidically effected compensation. This compensation is based on the action of a throttle gap, located on the outer circumference of the rotor, which as a function of the axial rotor position either limits or enables the back flow on the side of the rotor facing away from the blades. In this version as well, there is the risk of high shear stresses and the generation of flow stasis zones on the back side of the rotor.
U.S. Pat. No. 5,947,703 also describes an electromagnetically suspended centrifugal pump. Here, the drive of a covered impeller is effected by means of a unilaterally axially engaging permanent-magnet face-end rotary coupling or by an encircling stator field, whose forces of attraction cause the pump rotor at the housing to strike the wall unless the axial rotor position is regulated by a sensor-based active electromagnetic feedback. If this regulation fails, mechanical emergency bearings in the form of end journal bearings, slide bearings, point bearings, and hydrodynamic pressure bearings are provided, which are meant to prevent a life-threatening seizing of the pump rotor. All these proposals share the disadvantage of mechanical wall contact between the rotor and the housing, with the known consequences of damage to the blood.
International Patent Disclosure WO 01/42653 A1 describes a centrifugal pump with electromagnetic active position regulation of the pump rotor in all six degrees of freedom in space; the position, speed and acceleration of the rotor are not detected by sensors but derived from current signals of the active magnet bearings. This disadvantageously makes for an extremely complex mechanical construction of the rotor and multiple stators as well as extremely complex regulating electronics with an additional energy requirement, especially since to avoid high axial destabilizing forces, an ironless motor has to be used, which because of its poor efficiency heats up sharply.
The aforementioned disadvantages of active electromagnet bearing of the pump rotor were the impetus for a number of inventions in which complicated sensors and electronics were meant to be eliminated by means of hydrodynamic stabilization of the rotor/impeller.
For instance, in U.S. Pat. No. 5,324,177 and International Patent Disclosure WO 01/72351 A2, a hydrodynamic support bearing are used for radial stabilization of the rotor of an electrical direct current machine, and it carries the open pump rotor. A disadvantage here is the long axial length of the narrow, eccentric bearing gap, in which high shear stresses are operative, and which for being washed out requires auxiliary blades and a purging circuit from the high- to the low-pressure side of the pump. This arrangement involves the familiar risks of high shear and inadequate heat dissipation, which lead to traumatization of the blood.
These disadvantages are partly avoided in U.S. Pat. No. 6,227,797. In it, in a rotationally symmetrical housing, the pump rotor is embodied such that its surfaces on all sides form wedge-shaped gaps relative to the housing, in the direction of the active faces inclined in the direction of the relative motion. The pump rotor and housing thus form a hydrodynamic three-dimensional slide bearing, as is entirely usual in mechanical engineering. The supporting fluid film of blood, which acts as a lubricant for these wedge-shaped faces, covers a large area and especially at the circumference of the rotor is subjected to high shear stress, for which typical values of 220 N/m2 are given. This shear stress is thus within a range in which damage to blood cells, especially thrombocytes, from shear must be feared. Other disadvantages of this version are that the open pump rotor is surrounded on all sides relative to the housing by narrow gaps, in which high viscous friction prevails. The necessity of splitting the rotor into segmental blocks, to allow the blood to pass from the inlet to the outlet of the pump, stands in the way of optimizing the fluid-mechanical efficiency of the pump. Accordingly, for an implantable blood pump with low energy consumption, which is a worthwhile goal, the stated hydraulic degrees of efficiency of at most 11% are prohibitively low. The long axial length of the rotor moreover causes hydrodynamic radial shear on the rotor, which can necessitate a split spiral conduit, which favors the development of thromboses. Moreover, the housing is complicated to manufacture. The embodiment of a covered pump rotor shown in FIG. 20, with a surface structured in wedgelike shape in sectors, does not overcome these disadvantages, especially since it cannot be seen what path the blood is supposed to take to flow through such a rotor.
A quite similar version of hydrodynamic axial stabilization of an open pump rotor by means of floating wedge-shaped faces inclined in the direction of rotation is described in International Patent Disclosure WO 00/32256. Once again, the disadvantages are damage to the blood and a complicated housing construction. The radial centering of the rotor is moreover done here not by hydrodynamic but rather by permanent magnet reluctance forces of a face-end rotary coupling or of an electromagnetic drive motor.
WO 99/01663 discloses a hydraulically suspended pump rotor, which is meant to float by Archimedes buoyancy, since it has the same density as the fluid to be pumped. This pump must be embodied with two inlets, or the inflow must be diverted inside the pump by 180°; the result is large wetted internal surfaces as well as questionable hydrodynamic stability.
WO 01/70300, for hydrodynamic stabilization, proposes a conical rotor with slitlike openings for the flow to pass through and guide faces, through which a fluid flow oriented counter to the housing is generated that is meant to have a stabilizing effect. If that does not suffice, an active magnet bearing is provided for radial stabilization, but this represents additional electronic complication and expense. In a number of patents (WO 00/32257, WO 00/64508, EP 1 027 898 A1, and U.S. Pat. No. 5,840,070), combinations of the most various principles are employed for stabilizing the pump rotor: ball-spur bearings, passive permanent magnet radial bearings, active-sensor-based electromagnetic axial bearings, hydrodynamic wedge-shaped face bearings with both an axial and a radial action, supplemented by such auxiliary constructs as profiling of the rotor and/or of the housing by means of overlays, ribs, and disks, conduits, and other provisions.
It is notable that at least three of these principles must always be employed in combination in order to assure contactless rotation of the impeller in the pump, and that in the wedge-shaped face bearings, given the stated gap width of approximately 0.013 to 0.038 mm, shear stresses (of over 600 N/m2) occur, which are highly likely to damage blood.
A critical assessment of the prior art discussed consequently shows that the contactless rotation of the rotor of a centrifugal pump in the housing is attained either by means of high complexity and expense for sensors and electromagnetic regulation, or at the cost of a high hydrodynamic load on the blood from damaging shear stresses.